1,25-dihydroxyvitamin D.sub.3 ("1,25-(OH).sub.2 D.sub.3 "), the hormonal form of vitamin D, has several important biological activities in mammals. These activities include: (i) stimulation of intestinal calcium and phosphate transport from the lumen of the small intestine to the plasma; (ii) mobilization of calcium from bone to plasma; and (iii) reabsorption of calcium in the distal renal tubule. The biological activities of vitamin D ultimately lead to the elevation of plasma calcium and phosphorus levels which are necessary for bone mineralization and proper neuromuscular function.
The biological activities of 1,25-(OH).sub.2 D.sub.3 are mediated via intracellular receptor protein. DeLuca, H.F. et al., (1983) Ann. Rev. Biochem. 52, 411-439. (The disclosures of all articles recited herein are incorporated by reference as if fully set forth below.) The probable mechanism by which 1,25-(OH).sub.2 D.sub.3 elicits the intestinal calcium and phosphorus transport response consists of the 1,25-(OH).sub.2 D.sub.3 hormone entering the target cell and binding the nuclear receptor. The interaction of hormone with receptor may introduce changes in receptor conformation that allow the receptor to interact with chromatin. This interaction alters the expression of genes whose protein products influence functions such as calcium transport and mobilization. Link, R. et al. (1985) in The Vitamin D Receptor, Academic Press, New York, pp. 1-35.
Vitamin D-dependent rickets Type II is a disease that exemplifies the receptor-dependent function of 1,25-(OH).sub.2 D.sub.3. Bell, N.H. et al. (1978) N. Engl. J. Med. 298, 996-999. Patients with this disease suffer from hypocalcemia despite having elevated levels of 1,25-(OH).sub.2 D.sub.3 in their plasma because they have a target organ resistance to the hormonal derivative of vitamin D. A defect in the 1,25-(OH).sub.2 D.sub.3 receptor exists in at least one subgroup of rickets Type II patients. Eil, C., et al. (1986) Adv. Exp. Med. Biol. 196, 407-422.
The sequences for various animal vitamin D receptors are known. See McDonnell, D.P. et al., (1987) Science 235, 1214-1217 (Avian) (SEQID NO:3) Burmester, J.K., et al. (1988) Proc. Natl. Acad. Sci. USA 85, 1005-1009 (rat) (SEQ ID NO:4), and Baker, A.R. et al., (1988) Proc. Natl. Acad. Sci. USA 85, 3294-3298 (human)(SEQID NO:5). The amino acid sequences for these cDNAs have much in common. For example, the cDNAs display a cysteine-rich region at the amino terminus, characteristic of a DNA binding region. Also, the hydrophobic amino acids near the carboxy terminus form what is likely the hydrophobic pocket responsible for hormone binding. Domains within the human 1,25-(OH).sub.2 D.sub.3 receptor protein have been defined more precisely in McDonnell, D.P. et al., (1989) Mol. Endocrinol. 3, 635-644.
Discovery of cis-acting vitamin D-response elements (DRE) lying within the upstream regions of the human (Kerner, S.A., et al. (1989) Proc. Natl. Acad. Sci. USA 86, 4455-4459) and rat (Demay, M.B., et al. (1990) Proc. Natl. Acad. Sci. USA 87, 369-373, Markose, E.R. et al., (1990) Proc. Natl. Acad. Sci. USA 87, 1701-1705) osteocalcin genes, and the mouse osteopontin gene (Noda, M. et al., (1990) Proc. Natl. Acad. Sci. USA 87, 9995-9999) is consistent with 1,25-(OH).sub.2 D.sub.3 being a member of the steroid family of receptors. The sequences of other receptor DNA can now readily be determined using the existing sequences as hybridization probes against genomic and CDNA libraries, obtaining the cDNA using standard screening techniques, and then sequencing the DNA.
Lack of a low cost source of large amounts of 1,25-(OH).sub.2 D.sub.3 receptor has hindered commercial use and scientific studies of the receptor. For example, as disclosed in U.S. Pat. No. 4,816,417, 1,25-(OH).sub.2 D.sub.3 receptor is useful in an assay for 1,25-(OH).sub.2 D.sub.3.
Isolation-of receptor from natural animal cells produces only very small quantities at very great cost. Dame, M. et al., (1986) Biochemistry, 25, 4523. The human 1,25-(OH).sub.2 D.sub.3 receptor has been expressed from full length DNA in Saccharomyces cerevisiae (yeast) cells. Sone, T., et al., (1990) J. Biol. Chem. 265, 21997-22003. However, the authors indicated potential problems in that the recombinant product upon purification lacked activities comparable to the natural receptor protein and amounts of receptor protein produced were quite low.
The need therefore exists for the creation of an improved method for expressing vitamin D receptor protein.